Epstein-Barr virus (EBV) is associated with both B cell cancers (Hodgkin lymphoma, Burkitt lymphoma) and epithelial cell cancers (nasopharyngeal carcinoma, gastric carcinoma). EBV infection of B cells triggers activation of several signaling pathways that are critical for cell survival, virus latency, and growth transformation. To identify EBV proteins important for regulating cell signaling, we used a proteomic approach to identify viral proteins that activate transcription factor (AP-1, NF-B) promoters. We found that EBV BGLF2 protein activated the AP-1, but not NF-B, promoter. Expression of EBV BGLF2 in cells activated p38 and c-Jun N-terminal kinase (JNK), both of which are important for mitogen-activated protein kinase (MAPK) signaling. Deletion of the carboxyl-terminal 66 amino acids of EBV BGLF2 reduced the ability of BGLF2 to activate JNK and p38. Expression of EBV BGLF2 enhanced EBV BZLF1 (a viral protein that switches the virus from latency to lytic replication) expression in latently EBV-infected lymphoblastoid cell lines, and knockdown of BGLF2 reduced EBV reactivation induced by IgG cross-linking. Expression of BGLF2 induced BZLF1 expression and virus production in EBV-infected gastric carcinoma cells. BGLF2 enhanced BZLF1 expression and EBV production by activating p38; chemical inhibition of p38 and MAPK/ERK kinases 1 and 2 (MEK1/2) reduced expression of BZLF1 and virus production induced by BGLF2. In summary, the EBV tegument protein BGLF2, which is delivered to the cell at the onset of virus infection, activates the AP-1 pathway and enhances EBV reactivation and virus production. We described a patient who was presented with periodic fever which was thought to be caused by Epstein-Barr virus. He returned to clinic 22 years later and was found to have an elevated level of IgD, mutations in the mevalonate kinase gene, and hyperimmunoglobulin D syndrome was diagnosed. Thus, our patient's elevated EBV level was probably due to impaired immune surveillance and not the cause of his inflammatory condition. Therefore, it is important not to assume that EBV is the cause of an unexplained inflammatory disorder in the absence of a thorough evaluation, including genetic testing, for other potential causes.